Surface Acoustic Wave (SAW) sensor is a kind of sensor in which the SAW acts as an environment dependent information carrier. This kind of sensor has properties of high precision, fast response, small size, etc., and is widely applied in fields of aerospace, food hygiene, environmental monitoring, pharmaceutical chemicals, process monitoring, military security, etc., in which SAW sensor displays its unique technical advantages.
Currently, a variety of SAW multi-parameter sensors have been disclosed. For example, Chinese Utility Model CN201837405U disclosed a backscattering surface acoustic wave sensor with pulsed excitation, wherein wireless pulse signals emitted by a remote sensing device are received by a sensor antenna and drives an interdigital transducer to generate radio frequency surface acoustic wave. The acoustic wave pulse is reflected by a reflector and reversely reflected to the remote sensing device by the connected antenna. Temperature and other sensitive variations are extracted during detection of changes in reflection i.e. the delay in the sensor. Chinese patent application CN102313614 disclosed a surface acoustic wave thermometer with improved accuracy, wherein a plurality of transmitters are used to generate backscattering pulse sequences with time intervals, and the temperature detection accuracy is improved by combining time delay differences among those sequences of pulses and phase differences of a plurality of pulses at a single frequency. The patents described above employ a SAW device comprising piezoelectric materials. It should be noted that a drift will appear in backscattering time delay with the aging of piezoelectric materials or electrical connectors, which affects the long-term stability of the measurement. Meanwhile, the structure comprises delay line of larger size, and thus is uneasy to be miniaturized. Also, reliable wireless communication distance is reduced since such high path loss has been introduced.
In addition, China Patent CN201348624Y further disclosed a multi-channel surface acoustic wave chemical sensor, wherein such sensor comprises a plurality of parallel surface acoustic wave detecting channels and a reference channel. Frequency difference between detecting channels and the reference channel is generated by a frequency mixer and delivered to an analyzing device via cables. The invention needs to deliver frequency domain differential signals at low frequency via cable connection, and contains an active peripheral circuit, making it impossible to obtain a passive wireless sensor. Further, the sensor does not include function(s) of temperature or humidity measurement.